The introduction of ceramic orthodontic brackets, made from single crystal or polycrystalline sapphire is seen as a major advancement in the aesthetics of orthodontic treatment, compared to the existing stainless steel brackets. Unfortunately, there are problems with the ceramic systems. The most serious of the drawbacks occurs during the bracket removal after orthodontic treatment is complete.
Several problems occur during the removal process. Higher forces are required to remove ceramic brackets than the metal brackets from the teeth. This is attributed to the peeling mechanism used to remove the metal bracket that is not available for the ceramic bracket. In the ceramic system, all three components (the bracket, the enamel, and the highly ceramic filled polymer resin) are strong and brittle. These higher forces have at times, exceeded the strength of either the bracket itself, or more importantly the enamel to which the bracket is bonded. If the bracket fractures, diamond drilling of the residual ceramic is required for removal. If the enamel fractures, an expensive restorative dental procedure is required to repair the fractured region. Either procedure is time consuming and stressful for both the patient and the dentist. "Enamel crazing" has been reported as an additional sign of the brittle removal of these brackets. While crazing does not lead to an immediate need for restorative care, it does indicate enamel damage. There is a real need to make the removal process easier and more predictable.
There have been a number of efforts aimed at facilitating the removal process. Bracket manufacturers have attempted to place deliberate flaws within the brackets base to cause a lower strength failure within this region during removal. However, placing stable flaws within the ceramic is not trivial or inexpensive. Other efforts have looked at different removal techniques (i.e. torsional and shear modes). Another technique under development involves the use of a heated tool to lower the modulus and tensile strength of the adhesive during removal, but considering the potential tooth pulp damage while heating the adhesive, widespread use of these instruments has not developed.
In order to solve the problems with the prior art solutions to the problem, a modification of the polymeric adhesive mechanical properties has been achieved through a controlled interaction with plasticizers, thereby selectively permitting control over the removal forces by predicating the failure mode of the adhesive as being a ductile failure mode rather than a brittle failure mode. cl SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided an orthodontic adhesive wherein the modulus of the filled ceramic adhesive can be tailored through the use of selected plasticizers with likely FDA approval.
It is an object of this invention to provide an adhesive wherein increases in the plasticizer content of the adhesive lead to lower measured adhesion values when bonded to a standard substrate.
It is still another object of this invention to make removal of the more aesthetically pleasing ceramic brackets easier and more predictable through a ductile failure mechanism within the adhesive, rather than the current brittle mechanism.
These and other objects of this invention will be evident when viewed in light of the drawings, detailed description, and appended claims.